如何使用 Python+PyGObject 的 GObject.bind_property 函数进行 2 路数据绑定？

Question

这个问题的背景（以及我的总体目标）是以一种很好的方式构建 Python GTK 应用程序。我正在尝试使用GTK's bidirectional data bindings 将小部件属性绑定到模型属性。

我的期望是双向绑定应该保持两个属性同步。相反，我发现更改仅在一个方向传播，即使我使用的是GObject.BindingFlags.BIDIRECTIONAL 标志。我创建了以下最小示例和失败的测试用例test_widget_syncs_to_model 来说明问题。请注意，在更实际的示例中，模型对象可以是Gtk.Application 的实例，而小部件对象可以是Gtk.Entry 的实例。

import gi

gi.require_version('Gtk', '3.0')
from gi.repository import Gtk, GObject
import unittest


class Obj(GObject.Object):
    """A very simple GObject with a `txt` property."""

    name = "default"
    txt = GObject.Property(type=str, default="default")

    def __init__(self, name, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.name = name
        self.connect("notify", self.log)

    def log(self, source, parameter_name):
        print(
            f"The '{self.name}' object received a notify event, "
            f"its txt now is '{self.txt}'."
        )


class TestBindings(unittest.TestCase):
    def setUp(self):
        """Sets up a bidirectional binding between a model and a widget"""
        print(f"\n\n{self.id()}")
        self.model = Obj("model")
        self.widget = Obj("widget")
        self.model.bind_property(
            "txt", self.widget, "txt", flags=GObject.BindingFlags.BIDIRECTIONAL
        )

    @unittest.skip("suceeds")
    def test_properties_are_found(self):
        """Verifies that the `txt` properties are correctly set up."""
        for obj in [self.model, self.widget]:
            self.assertIsNotNone(obj.find_property("txt"))

    @unittest.skip("suceeds")
    def test_model_syncs_to_widget(self, data="hello"):
        """Verifies that model changes propagate to the widget"""
        self.model.txt = data
        self.assertEqual(self.widget.txt, data)

    def test_widget_syncs_to_model(self, data="world"):
        """Verifies that widget changes propagate back into the model"""
        self.widget.txt = data
        self.assertEqual(self.widget.txt, data)  # SUCCEEDS
        self.assertEqual(self.model.txt, data)  # FAILS


if __name__ == "__main__":
    unittest.main()

以上程序输出：

ssF
======================================================================
FAIL: test_widget_syncs_to_model (__main__.TestBindings)
Verifies that widget changes propagate back into the model
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/jh/.config/JetBrains/PyCharmCE2021.1/scratches/scratch_14.py", line 52, in test_widget_syncs_to_model
    self.assertEqual(self.model.txt, data)  # FAILS
AssertionError: 'default' != 'world'
- default
+ world


----------------------------------------------------------------------
Ran 3 tests in 0.001s

FAILED (failures=1, skipped=2)


__main__.TestBindings.test_widget_syncs_to_model
The 'widget' object received a notify event, its txt now is 'world'.

Process finished with exit code 1

我的具体问题是，我怎样才能让双向数据绑定工作？...如果有人可以修复我的示例或提供另一个工作示例，我会很高兴。

从更广泛的意义上说，双向绑定是在结构良好的 Python GTK 应用程序中同步 UI 状态和模型状态的方式吗？这样做的预期和良好支持的方式是什么？谢谢！

Answer 1

我在gnome discourse thread about bidirectional property bindings in python 得到了答复。

为了更清楚，下面的代码确实不工作，因为标志没有正确传递：

# broken, flags are passed incorrectly as keywords argument:
self.model.bind_property("txt", self.widget, "txt", flags=GObject.BindingFlags.BIDIRECTIONAL)

相反，标志必须按如下方式传递：

# functioning, flags are passed correctly as a positional argument:
self.model.bind_property("txt", self.widget, "txt", GObject.BindingFlags.BIDIRECTIONAL)

更多示例代码：例如，test_bidirectional_binding test case 中的 pygobject 源代码中演示了正确使用双向绑定。